Cylinder Liner Wear Monitoring: Methods and Limits

Cylinder liner wear on a large slow-speed two-stroke engine is not a single phenomenon but a competition between three distinct mechanisms, each driven by different physical and chemical conditions. Abrasive wear from catalyst fines in residual fuel, corrosive wear from sulphuric acid condensation at low loads, and adhesive wear during boundary-lubrication breakdown all leave recognisably different fingerprints on the bore surface and in the oil samples that drain from it. The monitoring task is to detect which mechanism dominates in any given engine, cylinder, and operating regime, and to act before the fingerprint becomes a failure. This article covers the mechanisms, the measurement methods, the specific limits published by MAN Energy Solutions and WinGD, and the operational decisions those measurements drive. For the physical design of liners themselves, including material grades and porting geometry, see the companion article on cylinder liner design for two-stroke engines. For the lubrication hardware that delivers cylinder oil to the bore, see cylinder lubrication systems for two-stroke engines. For the relationship between cylinder oil alkalinity and fuel sulphur that underpins corrosion control, see cylinder oil base number and fuel sulphur.

Use the cylinder liner wear rate calculator to benchmark measured bore growth against published limits and flag cylinders that need attention.

Wear mechanisms

Abrasive wear from catalyst fines

Abrasive wear is caused by hard particles trapped between the liner surface and the ring faces. On modern two-stroke engines the dominant abrasive source is catalyst fines, the aluminium silicate residue left in heavy residual fuel after the catalytic cracking process. ISO 8217:2017 sets the delivery limit at 60 mg/kg total aluminium plus silicon content in the fuel as received aboard, but monitoring programmes run by class societies and flag-state port-state control routinely find bunker samples that exceed this threshold. Particles below about 10 microns in diameter pass through the oil film without significant cutting action; particles in the 10 to 25 micron range match the ring-to-liner film thickness and act as lapping compound. Particles above 25 microns can score the liner surface directly.

CIMAC Working Group 7 guidance documents the mechanism in detail. A well-operated purification chain, settling tank at 50 to 60 degrees Celsius for a minimum of 24 hours, followed by two-stage centrifuging, can reduce cat-fines concentration to below 15 mg/kg at the service tank, but that requires the purifiers to be correctly set for the specific fuel density and temperature. Any overflow from a settling tank, or purifier operation at excessive throughput or wrong temperature, passes contaminated fuel forward. The scrape-down oil sample then shows a rising iron trend without the other indicators of cold corrosion or scuffing.

The wear pattern from abrasive cat-fines is relatively uniform along the liner axis, in contrast to the top-of-stroke concentration seen with cold corrosion. Scoring lines visible through the scavenge ports run axially rather than in the helical pattern left by ring contact at angles.

Cold corrosion from sulphuric acid

Cold corrosion is the attack of the cast iron liner surface by condensed sulphuric acid, and for most slow-speed two-stroke operators it has been the dominant wear mechanism since 2020 when MARPOL Annex VI Regulation 14 drove a widespread shift from high-sulphur heavy fuel oil to very low sulphur fuel oil at 0.50% sulphur and marine gas oil at 0.10% sulphur. The acid forms when combustion gases containing sulphur dioxide and sulphur trioxide contact water vapour and condense on surfaces below the acid dew point. The dew point of sulphuric acid in diesel combustion gases lies roughly between 140 and 160 degrees Celsius depending on sulphur content and local partial pressures.

Two operational conditions drive liner surface temperatures below that dew point. First, low engine load: at part load the heat release per cycle falls, piston and liner temperatures drop, and the cooling system may remove proportionally too much heat. At continuous loads below 25% of maximum continuous rating the liner surface in the upper stroke can fall below the acid dew point even on engines that maintain acceptable temperatures at higher loads. Second, a cooling water set-point that is too low for the fuel sulphur and ambient conditions. Several engine manufacturers, including MAN Energy Solutions in their service letters on the Alpha Lubricator system, recommend raising jacket cooling water outlet temperature to 85 to 95 degrees Celsius and reducing heat input to the charge air cooler when engines operate at low loads on low-sulphur fuels.

Once acid condenses, it attacks the liner surface at rates that can reach 0.1 to 0.5 mm/1,000 h when conditions are severe, compared to the 0.03 to 0.08 mm/1,000 h baseline for well-managed engines. The attacked surface appears rough and dark, with a characteristic pitted texture concentrated in the upper bore above the scavenge ports. The iron in scrape-down oil samples rises, but the chromium from ring face coatings rises less than in abrasive wear, because acid attack preferentially removes the cast iron matrix rather than the harder ring surfaces.

Adhesive wear and scuffing

Adhesive wear, which in severe form becomes scuffing or micro-seizure, occurs when the hydrodynamic oil film between the ring and liner breaks down and the asperities on the two metal surfaces make direct contact. The contact generates local temperatures high enough to weld and then tear the asperities, removing material from both surfaces. Mild adhesive wear occurs continuously in normal operation, particularly near top dead centre where ring velocity is lowest and film thickness is thinnest. Catastrophic scuffing is a sudden event: the ring face fuses to the liner over a patch of several centimetres, the ring may stick in the groove, and the engine will produce an audible knock or show a sharp drop in peak firing pressure on the affected cylinder indicator card.

The conditions that trigger scuffing are insufficient cylinder oil supply at the moment of maximum pressure, excessive ring face temperature from hot spots in the liner cooling, contaminated or degraded cylinder oil, or rings that have lost their running geometry through prior wear. On engines fitted with electronically controlled cylinder lubrication systems such as the MAN Alpha Lubricator or the WinGD Lubtronic system, a sensor or logic failure that interrupts oil injection during the high-pressure phase can produce scuffing in one to two engine revolutions. The scrape-down sample following a scuffing event shows an iron spike of several hundred mg/kg in a single sample rather than the gradual trend rise that characterises abrasive or corrosive wear.

Wear rate reference values

MAN Energy Solutions service documentation for large bore ME series engines defines three wear rate tiers based on operational experience across the fleet:

where $r_{\text{current}}$ is the measured bore radius at the current inspection, $r_{\text{baseline}}$ is the radius at the commissioning or last liner installation measurement, and $\Delta t$ is the elapsed operating hours in thousands.

WinGD guidance for the X-series (formerly RT-flex) engines is consistent with MAN’s tiers, with the alarm threshold placed at 0.8 to 1.0 mm per 1,000 hours for acute events detected through scrape-down iron analysis, which reflects a different reference basis (drip oil iron concentration in mg/kg rather than direct bore measurement). Those two representations of the same underlying phenomenon are related by the liner surface area and the oil flow rate, but they cannot be converted to a single universal figure because the relationship depends on engine geometry and sampling methodology.

Wear limits and renewal criteria

MAN Energy Solutions sets the maximum acceptable total bore wear at 0.8% of the nominal bore diameter, measured as the increase in the bore radius from the original commissioning value. WinGD applies a similar criterion. For representative bore sizes:

These figures are diametric wear limits expressed as radius change; class society survey records report bore diameter growth, and the limit must be halved to compare with a radius measurement. At wear approaching 0.6% of bore diameter, most OEM guidance recommends scheduling liner renewal at the next convenient port call. At 0.8% the renewal is mandatory before the engine returns to sea service.

Bore ovality, the difference between the maximum and minimum diameter at a single axial position, has a separate limit. MAN guidance typically sets the maximum acceptable ovality at 0.10 to 0.15 mm for large bore engines. Ovality above this figure indicates uneven wear that will eventually prevent the piston rings from maintaining a continuous gas seal.

Measurement methods

Direct bore measurement through the scavenge ports

The standard in-service bore measurement method uses a calibrated internal bore gauge inserted through the scavenge ports when the piston is positioned at top dead centre, making the upper liner accessible. The technique requires no disassembly beyond opening the scavenge inspection covers, which are present on all uniflow scavenged engines and accessible in port or at anchor.

The bore gauge contacts the liner at three or more circumferential positions at each axial measuring plane. Modern digital gauges transmit readings directly to a data logger. IMO Class survey records and OEM maintenance manuals specify the measurement planes: typically four axial positions between the top ring TDC contact area and the upper scavenge port edge, plus an additional position in the mid-stroke zone when the piston is repositioned to BDC. The measurement at TDC reveals the ring contact area that experiences the highest pressure and lowest sliding velocity, the zone most susceptible to adhesive wear and cold corrosion. The measurement near the scavenge port edge captures the port-belt corrosion pattern characteristic of cold corrosion under low-load slow-steaming operation.

Each axial position is measured in at least two perpendicular directions: fore-aft and port-starboard on four-stroke trunk engines, and aligned with and perpendicular to the crankshaft axis on two-stroke crosshead engines. The difference between the two readings at the same plane is the ovality at that position.

Scrape-down oil analysis

Scrape-down sampling is the primary continuous monitoring tool between direct bore measurements. As the piston descends through the scavenge ports, a small quantity of oil is scraped off the liner wall by the lowest oil scraper ring and by the port edges. This oil collects in the scavenge drain and can be captured with a sampling bottle within a few minutes after the piston has passed. The sample represents the accumulated condition of the liner surface in the upper stroke over the preceding hours of operation.

The sample is sent to a shore laboratory for inductively coupled plasma (ICP) spectroscopy or atomic emission spectroscopy. The key results for liner monitoring:

• Iron (Fe) concentration, mg/kg: the primary indicator of liner wall wear. Values of 50 to 200 mg/kg are typical for well-managed engines; values above 400 mg/kg indicate elevated wear; values above 1,000 mg/kg indicate acute wear or a scuffing event.
• Chromium (Cr) concentration, mg/kg: indicates ring face wear. Modern rings with chromium-ceramic or plasma-sprayed coatings contribute chromium when the coating wears. Rising chromium alongside rising iron suggests simultaneous liner and ring wear, consistent with abrasive conditions.
• Aluminium plus silicon (Al + Si), mg/kg: indicates cat-fines present in the oil reaching the liner. Values above 30 mg/kg in scrape-down samples suggest cat-fine contamination of the cylinder oil system despite fuel purification.
• Residual base number (BN) of the scrape-down sample: indicates how much alkalinity the cylinder oil retained after neutralising combustion acid. A scrape-down BN below 25 in a 40-BN oil suggests the feed rate is insufficient for the acid load, or the oil grade is mismatched to the fuel sulphur. MAN Service Letter SL2021-693 on Alpha Lubricator operation provides the specific BN depletion targets for different operating modes.

Shipboard programmes typically take one scrape-down sample per cylinder per week at sea and after every significant load change. The sample result is plotted against the cylinder’s own historical trend, not against a fleet-wide average, because individual cylinder characteristics (cooling geometry, ring condition, injection timing) affect the baseline.

Borescope and visual inspection at scavenge port access

Visual inspection complements the quantitative methods. The scavenge port covers give access to the lower two-thirds of the liner with the piston at TDC. A rigid or flexible borescope equipped with a camera reaches the upper bore above the ports. The inspection focuses on:

Bore polishing: a mirror-bright circumferential band where the crosshatch honing pattern from the last liner installation or reconditioning has been worn away. Some polishing in the top ring zone is normal and expected after the first few thousand hours of service. Polishing that extends more than 30% of the ring travel distance, or polishing with no visible crosshatch anywhere in the ring zone, indicates that the hydrodynamic oil film is failing to separate the ring and liner surfaces adequately. The usual causes are too-low cylinder oil feed rate, wrong oil viscosity for the operating temperature, or a ring face geometry mismatch.

Corrosion patches: dark, rough, pitted areas distinct from normal wear. Above the ports, concentrated in a band corresponding to the area of the liner that sits at the top ring TDC position during low-load operation, cold corrosion patches look characteristically different from scoring. They spread laterally across the bore surface rather than running axially, and the surface texture is granular rather than grooved.

Scoring and axial grooves: parallel lines running in the direction of piston travel. Single fine lines from a particle that passed through indicate transient contamination. Multiple deep parallel grooves suggest sustained abrasive wear. Circumferential marks that cross the axial direction indicate ring-to-liner adhesion, the precursor to scuffing.

Lacquer and carbon deposits: hard varnish-like deposits on the liner surface indicate cylinder oil breakdown products accumulating. Excessive lacquer below the port belt suggests oil feed rate is too high for the operating temperature, or the oil is thermally unstable at that engine’s conditions.

All visible surface conditions should be photographed with the date, cylinder number, and axial position recorded. Comparison with the previous port’s photographs identifies slow-developing changes that single observations would miss.

Micro-gauging at major overhaul

When the liner is removed at a scheduled major overhaul, typically at 16,000 to 32,000 running hours depending on engine type and condition, a full dimensional survey is possible with the liner on a flat surface. A precision internal micrometer measures bore diameter at 10 to 15 axial positions, each in two perpendicular directions. The result is a complete wear profile: a graph of bore diameter against axial position that shows exactly where material has been lost and in which direction ovality has developed.

This profile, compared with the one taken at liner installation or at the previous overhaul, provides the total wear, the maximum wear rate point, and the ovality at each position. The data determine whether the liner can be returned to service after honing, or whether renewal is necessary. A liner with total wear below the renewal limit but significant ovality concentrated at one plane may still require renewal if the ovality cannot be corrected by honing.

Wear pattern interpretation

Different wear mechanisms leave different axial wear profiles:

Top-stroke concentration, falling toward mid-stroke: the standard pattern of normal mechanical wear, dominated by the elevated contact pressure and low sliding velocity near TDC. This pattern is expected and does not indicate a problem unless the magnitude exceeds the reference limits.

Upper bore concentration above the port belt with visible corrosion: the signature of cold corrosion. The acid condensation occurs where liner surface temperatures are lowest during combustion, which on a uniflow engine corresponds to the upper bore during the period between scavenge port closing and exhaust valve opening. The port belt itself may show less wear because the port edges interrupt the liner surface, and the oil drainage pattern differs there. This pattern is the main focus of wear analysis during slow-steaming operations because MARPOL Annex VI Regulation 14’s 0.50% sulphur cap drove fleets toward very low sulphur fuel oil at loads that may be below the liner’s thermal optimum.

Mid-stroke and lower bore concentration: unusual. Suggests a different mechanism, often contaminated cylinder oil reaching the liner in that zone, or a ring seating problem that causes the rings to contact the liner abnormally in mid-stroke.

Uniform wear along the full stroke: associated with abrasive contamination from cat-fines in the cylinder oil or from scavenge port deposit shedding. The abrasive acts uniformly across all contact zones because it is carried in the oil film regardless of position.

Localised wear at one axial band, circumferentially complete: points to a ring land problem, such as a broken or stuck ring contacting the liner at a fixed axial position. The bore gauge will show a sudden step in the axial wear profile at the affected plane.

Localized wear at one circumferential position: piston side thrust in large-bore engines creates higher contact pressure on the anti-thrust side. Fore-aft diameter exceeding port-starboard diameter at the TDC zone by more than 0.05 to 0.10 mm is within normal parameters; larger differences indicate structural distortion of the liner or its mounting.

The link between cylinder oil alkalinity, feed rate, and wear

The cylinder oil alkalinity reserve, expressed as base number (BN), is the primary chemical defence against cold corrosion. A BN-40 oil has twice the alkalinity reserve of a BN-20 oil per unit mass, but effective protection depends on both the BN and the feed rate: a BN-20 oil dosed at 1.2 g/kWh delivers the same total alkalinity per unit of fuel energy as a BN-40 oil at 0.6 g/kWh. MAN Energy Solutions introduced the Anti-Corrosion Control (ACC) concept in service letters issued around 2014, formalising the relationship between fuel sulphur content, engine load, and the required oil dosing to keep scrape-down BN above the minimum protective threshold.

The formula embedded in the MAN ACC feed rate calculator links oil feed rate to fuel sulphur and oil BN:

with a minimum feed floor of 0.6 g/kWh regardless of calculated result. At 0.5% sulphur VLSFO with a BN-40 oil this formula produces 0.26 × 0.5 × (70/40) = 0.228 g/kWh, below the 0.6 g/kWh floor, so the floor governs. That floor is not arbitrary: it represents the minimum oil quantity needed to maintain a continuous hydrodynamic film in the upper bore independent of acid neutralisation.

WinGD’s Lubtronic system applies a similar logic through the engine management system’s load-dependent dosing map. The WinGD LCD (Load Cycle Dosing) approach adjusts injection timing within the engine cycle so that oil reaches the upper liner during the period of maximum pressure rather than at a fixed crank angle, improving film coverage at TDC. Both approaches aim at the same outcome: keeping the scrape-down BN above a defined threshold, typically 25 to 30 for engines on low-sulphur fuel.

For the specific relationship between fuel sulphur content and the correct BN selection, the article on cylinder oil base number and fuel sulphur covers the full selection logic and the consequences of BN mismatch in both directions: too-high BN on low-sulphur fuel produces hard calcium carbonate deposits on the liner surface, which paradoxically accelerate abrasive wear; too-low BN allows acid attack.

Slow-steaming and cold corrosion mitigation

Slow steaming, continuous operation at loads below 50% MCR and in extreme cases below 20% MCR, became widespread after 2008 as bunker costs rose and has remained common under the Carbon Intensity Indicator framework introduced by IMO. At low loads, several factors combine to increase cold corrosion risk:

The exhaust temperature falls, reducing the heat available to maintain liner surface temperatures above the acid dew point. The charge air temperature and pressure both fall, reducing the work done on the gas and the heat transferred to the liner through combustion. On many engine types, the cooling water flow rate does not reduce proportionally with load, so the cooling side removes a larger fraction of the available heat per cycle. The net effect is that a liner designed to run at 80 to 90 degrees Celsius at the upper bore surface at full load may fall to 60 to 70 degrees Celsius at 20% MCR, dropping through the acid dew point of typical VLSFO combustion products.

The operational countermeasures documented in MAN and WinGD service letters include:

Raising jacket cooling water outlet temperature to 85 to 95 degrees Celsius when operating below 40% MCR, using a thermostatic bypass to reduce the cooling water flow through the heat exchanger.

Running the engine at a higher load for one to two hours in every 24 to remove corrosion products and re-establish the protective oil film. MAN documentation on the Alpha Lubricator and ACC system describes this as a “cleaning run” or “high-load run,” typically to at least 75% MCR.

Using a higher-alkalinity cylinder oil grade during sustained low-load operation. Several operators transitioning from BN-40 to BN-70 or even BN-100 oils for slow-steaming voyages have reported scrape-down iron reductions of 30 to 60% compared with BN-40 at the same feed rate, based on fleet monitoring data published through class society technical bulletins.

Adjusting cylinder oil dosing upward at low loads rather than proportionally reducing it with the load-based dosing map. The ACC formula shows that required feed rate falls with falling sulphur content and rising BN, but the minimum floor remains 0.6 g/kWh; some operators apply a higher floor of 0.8 to 1.0 g/kWh during sustained slow steaming.

The MARPOL Annex VI Regulation 14 sulphur cap context matters here. Prior to January 2020 a ship burning 3.5% sulphur HFO required high-alkalinity oils and produced high corrosive wear rates when cold; after 2020 the same ship on 0.5% VLSFO or 0.1% MGO faces far lower sulphur acid loading but runs cooler liners and may not adjust its cooling system set-points. The risk has inverted: the cold corrosion danger on low-sulphur fuel is not from excess acid but from inadequate liner temperature, and the oil alkalinity required is much lower than in the HFO era.

Honing after liner renewal or reconditioning

A liner returned to service after reconditioning (grinding and re-honing to remove ovality or scoring) or a new replacement liner must be correctly honed before installation. The honing crosshatch pattern serves two functions: it provides a geometric texture that retains cylinder oil in the valleys between the hone marks, and it provides the asperities that the ring faces need to conform to the bore geometry during the running-in period. A plateau hone, in which a fine final honing step removes the sharpest peaks while preserving the oil-retaining valleys, is the current standard for MAN ME and WinGD X-series bore profiles.

The MAN recommended crosshatch angle for liner surfaces is 35 to 45 degrees from horizontal. Shallower angles retain more oil but slow the running-in period; steeper angles accelerate ring seating but may not retain sufficient oil for the first few hundred hours. The surface roughness after final plateau honing is specified as R_pk (reduced peak height) in the range 0.2 to 0.4 microns and R_k (core roughness depth) in the range 1.0 to 2.0 microns for modern thin-film lubricated bores.

Running-in procedures for new or reconditioned liners require elevated cylinder oil feed rates for the first 500 to 1,000 hours of service. MAN service documentation specifies increasing the ACC feed rate multiplier to 1.5 to 2.0 during the running-in period, then stepping it back to the normal calculated value after the bore surface shows a stable plateau polish pattern in borescope inspection. Scrape-down iron concentrations during running-in are expected to be higher than normal, in the range of 200 to 500 mg/kg, and should trend steadily downward within the first 500 hours.

Comparison of monitoring methods

Per-cylinder records and class survey integration

Maintaining the wear history

Every large slow-speed two-stroke engine should have a cylinder-level wear history that records, for each liner in service: the liner serial number and the bore diameter at installation, the measured bore diameter at each overhaul with the axial position and date, the accumulated running hours between measurements, and the calculated wear rate for each interval. Engine management software packages from major ship management companies maintain these records digitally, and some OEM service platforms, including MAN PrimeServ’s CEON (Condition and Efficiency Optimization Network) and WinGD’s equivalent telemetry services, pull scrape-down and indicator data directly from the ship’s engine monitoring system and trend it alongside fleet data for the same engine model.

The practical value of that per-cylinder history becomes clear when one cylinder’s wear rate diverges from its neighbours. On a six-cylinder engine where five cylinders show iron in the range of 80 to 120 mg/kg in scrape-down samples and one shows 350 mg/kg, the elevated cylinder’s history usually reveals the cause: a different overhaul date and thus different accumulated hours since honing, a scavenge fire that accelerated the previous ring wear, a change in the cylinder oil dosing valve setpoint that was not reset after maintenance, or a previous undocumented scoring event. Without the longitudinal record, the elevated sample looks like a fleet-wide event; with it, the investigation narrows to a single cylinder’s maintenance history.

Class society requirements

Class societies with machinery survey scope, including DNV, Lloyd’s Register, Bureau Veritas, ABS, ClassNK, Korean Register, and RINA, require documentary evidence of cylinder condition at major overhaul surveys. The exact requirements vary by society and survey scheme, but all require bore measurements at overhaul, photographic or written condition records for each liner, and a declared wear status relative to the OEM renewal limit. Under continuous survey schemes, which distribute the survey scope across a five-year cycle rather than concentrating it in a single drydock, individual cylinders may be presented for survey on different dates, and each presentation must show the complete wear history since the last class survey of that cylinder.

DNV’s CORIS (Condition Reporting and Inspection System) and Lloyd’s Register’s ShipRight maintenance programmes both have structured formats for recording cylinder wear data that satisfy the survey record requirements. Operators who maintain their data in a compatible format avoid re-measurement at survey because the ongoing records already demonstrate compliance.

A key requirement in most class society schemes is that the wear at survey must not exceed the OEM working limit (typically 0.6% of bore diameter) without an agreed plan for renewal. A liner at 0.7% bore wear found at survey triggers a condition of class: the liner must be renewed within a specified period, typically before the next port or before the next voyage depending on the margin above the limit and the trade. A liner already at or above 0.8% wear found at survey is grounds for requiring immediate renewal before departure. These thresholds align with MAN and WinGD’s own published guidance, so the OEM limits and the class requirements are consistent in practice.

Interaction with piston ring condition

Cylinder liner wear cannot be assessed in isolation from the condition of the piston rings that contact the liner surface. The wear on the ring face and the wear on the liner surface are driven by the same contact conditions, and the two components’ conditions reinforce each other: a worn ring with reduced face crown radius contacts the liner at a higher specific pressure, accelerating both ring face and liner wear; a scored liner with axial grooves damages ring faces by mechanical contact, producing iron and chromium wear metals simultaneously in scrape-down samples.

Ring-to-liner interaction is why the chromium-to-iron ratio in scrape-down samples carries diagnostic information. When iron rises but chromium stays stable, the liner wall is the primary wear source. When both rise together, the ring face coating is being damaged simultaneously. Modern ring face materials include:

• Thermally sprayed chromium ceramic (Cr2O3 with metallic ceramic matrix): the standard on MAN ME and WinGD X-series rings; very hard (approximately 1,000 to 1,200 HV), low friction coefficient, resistant to abrasive wear, sensitive to thermal shock from hot spots.
• HVOF (High-Velocity Oxygen Fuel) tungsten carbide coatings on premium rings: harder still, more expensive, used where cat-fines abrasion is severe.
• Gas-nitrided rings on some four-stroke engine applications: cheaper, adequate for medium-severity service.

Each coating has a characteristic wear rate and a different chromium-release signature in ICP analysis. A laboratory that knows the ring specification can distinguish chromium from ring face wear versus chromium from liner material (which contains no significant chromium), helping isolate the wear source.

The piston ring pack design for two-stroke engines article covers the ring face geometry, coating types, and ring-to-liner contact mechanics in detail. The wear monitoring perspective from this article and the design perspective from that one together define the complete tribological picture of the cylinder.

When a liner is returned to service after renewal or reconditioning, the ring pack condition at that point determines whether the running-in procedure succeeds. A new liner installed against worn rings with a collapsed face crown profile will not run in smoothly: the contact pressure distribution is wrong for the bore geometry, the oil distribution will be uneven, and the risk of adhesive contact in the first 500 hours is higher than with a matched new ring set. MAN and WinGD both recommend renewing the ring pack whenever the liner is renewed, and most planned maintenance systems flag the rings as a renewal item at the same overhaul interval.

Limitations

Direct bore measurement through the scavenge ports covers only the lower two-thirds of the liner stroke, and within that zone only the positions accessible with the bore gauge while the piston rests at TDC. The upper bore, where cold corrosion and TDC ring contact wear are most severe, can only be directly measured at major overhauls when the piston is removed. Operators relying solely on scrape-down oil analysis may see elevated iron and correctly identify accelerated wear without being able to pinpoint whether the wear is concentrated at the top or distributed along the stroke.

Scrape-down oil analysis is a lagging indicator. By the time a laboratory result returns aboard ship, the engine has run for days at the condition that produced the sample. An acute scuffing event can develop and worsen considerably before the sample result is available. Continuous on-board oil analysis systems that return results in hours rather than days are commercially available but not yet standard fit on most vessels in service.

The MAN and WinGD wear rate figures cited here, 0.03 to 0.08 mm/1,000 h typical and 0.8% of bore diameter maximum, apply to their respective engine families for large slow-speed two-stroke engines. Medium-speed four-stroke trunk-piston engines from other manufacturers operate to different limits, and applying two-stroke OEM figures to a four-stroke Wartsila or MAN medium-speed engine would be incorrect. The cylinder lubrication systems for two-stroke engines article clarifies the hardware differences between two-stroke crosshead and four-stroke trunk-piston configurations that make direct comparison of wear rates and limits between engine types unreliable.

Iron concentration in scrape-down samples varies with oil flow rate through the drain, which in turn depends on cylinder oil feed rate and engine load. Two engines with identical liner wear rates can produce different iron concentrations in their scrape-down samples if their feed rates differ. Trend analysis within a single engine, rather than absolute comparison against a fleet average, is the correct use of scrape-down data.

The acid-dew-point calculation for any specific fuel and engine configuration requires knowledge of local partial pressures of water vapour and sulphur oxide in the combustion gases. The values of 140 to 160 degrees Celsius used in OEM guidance documents are approximations for typical HFO and VLSFO combustion conditions. Actual dew points will differ for LNG dual-fuel engines, methanol engines, or ammonia engines currently in development, and the cold corrosion mitigation advice established for sulphur-based cold corrosion does not transfer directly to other fuel types.

See also

• Cylinder Liner Design for Two-Stroke Engines
• Cylinder Lubrication Systems for Two-Stroke Engines
• Cylinder Oil Base Number and Fuel Sulphur
• Cylinder Oil Feed Rate Optimisation
• Piston Ring Pack Design for Two-Stroke Engines
• Slow Steaming and Engine Cleanliness
• Bunker Quality and ISO 8217
• MARPOL Annex VI Regulation 14 Sulphur Cap
• Alpha Lubricator Electronic Cylinder Lubrication
• Cylinder Liner Wear Rate Calculator
• MAN ACC Cylinder Oil Feed Rate Calculator